1. Field of the Invention
The present invention relates to a signal transmission structure. More particularly, the present invention relates to a signal transmission structure and circuit substrate thereof capable of reducing insertion loss during signal transmission.
2. Description of the Related Art
A conventional circuit substrate comprises a multiple patterned circuit layers and a plurality of dielectric layers alternately laminated on top of each other. The patterned circuit layers are formed, for example, by patterning copper foils in photolithographic and etching processes. The dielectric layers are disposed between neighboring patterned circuit layers to isolate the patterned circuit layers. In addition, the laminated circuit layers are electrically connected to each other through plating through holes (PTH) or conductive vias. The method of forming a conductive via includes performing a sequence of photolithographic processes to form a hole in the dielectric layer and then filling the hole with a conductive material to form a conductive via for electrically connecting at least two patterned circuit layers. The method of forming a plated-through-hole includes performing a mechanical drilling process to form a through hole in the laminated circuit layers and the dielectric layers or a dielectric core and then performing an electroplating operation to form a plated layer on the interior wall of the hole for electrically connecting at least two patterned circuit layers.
FIG. 1 is a schematic cross-sectional view of a conventional circuit substrate. As shown in FIG. 1, the circuit substrate 100 comprises a lamination of six patterned circuit layers. The circuit substrate 100 has a laminated layer 110 comprising a top circuit structure 120, a core layer 130 and a bottom circuit structure 140. Wherein the top circuit structure 120 comprises a plurality of patterned circuit layers and at least a dielectric layer laminated sequentially over each other above the core layer 130. Furthermore, the via lands 124 and 126 of each patterned circuit layer are electrically connected through a conductive via 125. The bottom circuit structure 140 comprises a plurality of patterned circuit layers and at least a dielectric layer laminated sequentially over each other below the core layer 130. Furthermore, the via lands 144 and 146 of each patterned circuit layer are electrically connected through a conductive via 145. In addition, the core layer 130 has a plurality of through holes 132 that connect an upper surface 130a of the core layer 130 with a lower surface 130b of the core layer 130. The inner wall of each through hole 132 is completely covered by a conductive wall 134 such that the via land 126 of the top circuit structure 120 is electrically connected to the via land 146 of the bottom circuit structure 140. Moreover, a dielectric material 150 may fill the hollow portion within the conductive wall 134 and the through hole 132.
As shown in FIG. 1, the two via lands 126 and 146 disposed on the top surface 130a and the bottom surface 130b of the dielectric core layer 130 are completely covered both ends of a through hole 132. Furthermore, these via lands 126, 146 are electrically connected to each other through a hollow cylindrical conductive wall 134. Therefore, a signal on a signal line in the top circuit structure 120 can transmit in a downward direction sequentially through a connection pad 122, the conductive vias 123, 125 and the via lands 124, 126. After passing through the conductive wall 134 which is inside the through hole 132 in the core layer 130, the signal passes sequentially through the via lands 146, 144, the conductive vias 145, 143 and another connection pad 142 to be connected externally. The aforementioned conductive structures provide a signal transmission pathway linking two devices or two terminals (not shown).
Note that in the prior art the line width of the signal lines electrically connecting two devices or two terminals must be uniform. In other words, the electrical impedance in various parts of the signal line should be constant so that the characteristic impedance is kept at a constant value when electrical signals are transmitted along the signal line. In particular, for high speed and high frequency signal transmission, fine impedance matching design between two terminals of a signal transmission path is very important to reduce signal reflection which may occur because of impedance mismatch in the signal transmission path. In other words, reducing insertion loss and correspondingly increasing the dropping of return loss during signal transmission will prevent affecting signal transmission quality. However, the two via lands 126 and 146 in a conventional design cover an area larger than the hole area of the through hole 132. Furthermore, the conductive wall 134 completely covers the inner wall of the through hole 132 in the core layer 130 that alteration of a large impedance may occur when a signal passes through the relatively large area of the two via lands 126 and 146 and the hollow cylindrical conductive wall 134. Consequently, the impedance mismatch between the conductive wall 134 and the top circuit structure 120 or the bottom circuit structure 140 becomes more serious. Moreover, when a differential signal respectively flows through the conductive walls 134, 138 on the inner walls of two neighboring through holes 132 and 136, due to electromagnetic coupling between the two conductive walls 134, 138, switching a signal will interrupt another signal in one of the neighboring through holes 132, 126 and generate cross talk. Thus, signal transmission quality will be influenced.
To resolve the aforementioned problem, the distance between two neighboring through holes 132 and 136 has to be increased. In other words, the distance between two neighboring conductive walls is increased to avoid cross talk. However, this method requires larger layout area and hence cannot be applied to a limited or high-density area of circuit layout.
In addition, a differential signal transmitting amidst the signal transmission structure will change the current return circuit and the electric field when passing through the two hollow cylindrical conductive walls 134 and 138 due to the electromagnetic coupling between the conductive walls. Accordingly, a change in the characteristic impedance will occur. Hence, when the signal is transmitted to the second through holes 132, 136, signal reflection phenomenon will occur due to non-continuous impedance in the two conductive walls 134 and 138. As a result, signal transmission quality will be affected.
For transmission of a single signal or a differential signal, the higher the operating frequency, corresponding to a same operating frequency, the greater the return loss is when the signal passes the through hole 132, so that impedance mismatch becomes more serious. Furthermore, the higher the operating frequency, corresponding to a same operating frequency, the greater the insertion loss will be increased when the signal passes the through hole 132, so that impedance mismatch becomes more serious.
As described, a signal passing through a relatively expansive via lands 126, 146 and a hollow cylindrical conductive wall 134 having a relatively large cross-sectional large area, the characteristic impedance of the top circuit structure 120 and the bottom circuit structure 140 of the circuit substrate 100 will increase with frequency. Thus, the difference from originally designed impedance value will increase leading to an aggravation of impedance mismatch in the same signal transmission pathway.